In solid fuel fired furnaces, the fuel must be supported by a grate. The grate forms a means to support the burning fuel mass and to allow combustion air to be drawn up through the grate, to the burning mass, to aid combustion.
Several grates have been devised to perform this function. The simplest is a fixed grate which may be air or water cooled. Fixed grates, however, require that the firing rate of the solid mass be reduced or stopped to allow manual raking of the ash through the grates. Of course, while doing this, the boiler or heating system suffers, adversely affecting process and requiring much lost time in regaining operating temperatures once ash was removed. This also exposes the individual doing the raking to high temperature extremes from 20 minutes to an hour. Shutting down the system also allows cold air to contact refractory, creating a cold shocking condition which can cause spalling and checking of refractory which shortens operating life.
Traveling grates have been designed which move slowly across the furnace area picking up new fuel on one side and depositing the ash and other solid products of combustion on the far side of the furnace for removal. These grates are typically complicated, expensive and bulky. Traveling grates are also difficult to water cool.
Incline grates have also been used where the fuel enters the upper end, ignites, and burns while slowly sliding either by gravity or a mechanical means to an ash pit at the lower end.
Step grates combine several of the features of incline grates, wherein the burning fuel is jogged from one step to a lower step and eventually to an ash pit.
Still other grates have been devised which have hinged or pivoted portions which rotate or drop, thereby dropping the loose ash into a pit below the grate.
Another type of grate has built-in nozzles that are sequentially operated using air or steam to blast and direct the ash toward a lower ash pit for removal.
Many of the foregoing described grate systems have had limited success and are designed for very narrow usages. The grate design must take into account the way fuel is presented to the firing arrangement, how the firing affects the furnace, how the furnace configuration can rid itself of ash with interrupted or continuous firing and the type of solid fuel being fired and the amount of ash produced.
A need, therefore, exists for a grate for a solid fuel fired furnace which will support the burning fuel mass, allow air to circulate up through the burning fuel mass to support combustion, allow ash to fall through the grate to a collection area and crush or reduces the size of cinders, slag or clinkers which form beneath or within the burning fuel mass.